This invention is related to pulse rate measurement systems, and particularly to an improved pulse rate measuring system that provides a fast response for cancelling undesired noise signals and that rapidly and accurately develops pulse rate readings with minimum power and voltage level requirements.
In many systems, it is necessary to use a photodetector in ambient light to detect small pulses of light from a source, and to either determine the amplitude of the pulses, the rate of the pulses, or both the amplitude and rate of the pulses. One particular use of a photodetector in an ambient light situation is a pulsed infrared (IR) reflectance plethysmograph for heart rate measurement that may be integrated into a wristwatch or may be a medical instrument suitable for clinical use. The instrument in accordance with the invention is called a plethysmograph because it measures heart rate by measuring the change in size of the capillaries or blood vessels in a finger, for example, which change in size varies the reflectance characteristics of the capillaries to light energy (the capillaries dilate upon the arrival of the systolic pressure wave and the reflectance drops). Another use of a photodetector in high intensity ambient light may be an electronic game. For reliable operation of the plethysmograph, game, or the like, in ambient light, it is necessary to provide some means between the photodetector and the utilization system that has the ability to reject high levels of ambient light, even sunlight, to reject other undesired signals, and to maintain a fast response time.
In a plethysmograph system for measuring pulse rate when a finger is placed in a position so that the photodetector can sense reflected light pulses, the result is a temporary overload in the signal detection and amplification circuits. It is possible to sense this overload condition and use a feedback bias control to change the operating point of the phototransmitter or the photodetector to compensate for the background light level, but for fast response to 120 Hz background flourescent lighting this compensation must be done on a pulse-by-pulse basis. Another problem of a feedback bias control arrangement is overcorrecting to the point of suppressing the desired light pulses as well as the ambient light signal. This suppression of the desired light signal will normally occur with any electric feedback overload correction system which cannot distinguish between the ambient light components and the pulsed light. Under extreme overload conditions, this feedback bias control arrangement may require 20 to 30 seconds to stabilize to the point that accurate readings are provided, especially if the system is powered from a relatively low voltage power supply such as the 3 v battery supply of a digital wristwatch. Another possible solution of the overload problem comprises the generation of a reset command to be applied when the condition of overload is sensed.
An alternate approach to this problem of obtaining a useable signal in a relatively short time is to separate the desired light pulse signal from the unwanted background ambient light signal by amplifying the light pulse signal plus ambient signal, and converting it to a stored charge or voltage so that the amplitude of the unwanted ambient signal can be removed by a high level differencing (subtraction) circuit. This amplification of both the pulse signal plus the background signal prior to differencing, results in severe dynamic range limitations in those usual cases where the ambient light signal and amplitude is several thousand times greater than the pulsed light signal amplitude and where power supply voltages are limited such as in a digital wristwatch. Moreover, the process of determining the level of the ambient light signal to be subtracted may require 20 to 30 seconds to stabilize in a 3 volt system. A prior art pulse rate measuring system that amplifies the input signal is taught in U.S. Pat. No. 3,980,075, "Photoelectric Physiological Measuring Apparatus" by James E. Heule. Another prior art pulse rate measuring system in the form of a watch has been sold by the Pulsar Company and although the circuit arrangement is not known to me, it is believed that the watch does not respond satisfactorily in the presence of high amplitude ambient light such as direct sunlight. A pulse rate system that amplifies the photodetector current for providing a feedback signal to control the LED intensity and that utilizes a sample and hold circuit without prefiltering has been developed by others and requires 20 to 30 seconds to stabilize or else requires a separate control system which provides an initialization command upon sensing that no output signals have been produced for several seconds.
A system which requires 20 to 30 seconds to stabilize would not be acceptable without an initialization command for the plethysmograph where it is desirable to measure the heart rate immediately after exercise. If a delay of about 10 seconds is required before measurement can be made, the subject will have had some rest, enough to possibly allow the heart rate to decrease from the peak level. Also, in order to conserve time and provide timely data, a heart rate instrument for use by medical personnel should necessarily respond in much less time than 20 to 30 seconds. In all of these examples, as in others that may occur to those skilled in the art, it would be desirable to provide a light pulse detector system with ambient light rejection and other undesired signal rejection that automatically stabilizes in 2 to 3 seconds in order for it to be used for the intended purpose.